More than 18,000 people rode the MBTA system during the first weekend of late-night service, the T announced Tuesday — more than four times the average number of weekend riders who frequented the agency’s now-defunct Night Owl bus service in its first year.

But even as Massachusetts Bay Transportation Authority officials and transit advocates celebrated a successful debut weekend, the question remains: Will that be enough to keep the service alive beyond the end of the one-year pilot?

Preliminary data from the agency show that 10,017 people rode the subway system between 12:30 a.m. and 3 a.m. Friday night into Saturday morning, and 8,715 rode the subway the following night.

“It’s five times better than what we did with the Night Owl,” MBTA general manager Beverly A. Scott said. “That’s an apples-to-oranges comparison, but it’s already what our folks recognize as having been much more significant in ridership.”

The one-year pilot to extend weekend hours on the subway lines and the 15 most popular bus routes was announced last December, and is estimated to cost $16 million to $17 million — a small portion of that coming from private sponsors, including the Red Sox, Dunkin’ Donuts, Suffolk Construction, and the Boston Globe.

It’s a costly venture for the cash-strapped T, and transportation officials have warned that riders who want the service to remain alive must prove that there’s a steady demand; the Night Owl, for example, debuted to widespread enthusiasm in 2001 but saw ridership dwindle over the course of four years.

So officials are well aware that an impressive debut weekend does not guarantee the service’s success over the course of the next 51 weekends.

Scott has declined to say whether the T has any benchmarks for ridership numbers that will determine whether the pilot is a success.

But, she said, factors other than ridership will also be considered when assessing the results of the late-night experiment. Many of those factors, Scott said, will be anecdotal: The T will be monitoring whether the extended hours increase sales at local businesses, encourage restaurants close to T stations to stay open later, and make Boston’s convention centers more competitive in bringing high-profile events to the city.

“We’re clearly going to be looking at ridership . . . but when we look at, ‘What is the benefit of this?’ it’s a much bigger set of values,” Scott said. “If the sum total of whether this is considered to be successful is simply going to focus on ridership, I think we will be missing what it’s all about.”

Scott said she was optimistic the eagerness would remain high, but Stephanie Pollack, associate director of the Dukakis Center for Urban and Regional Policy at Northeastern University, was more skeptical, and raised the possibility that ridership might wane once the novelty wears off.

“You can’t draw too many conclusions from one weekend,” Pollack said.

“It was all over Twitter, all over social media . . . ‘Use it or lose it,’ that message clearly got out,” Pollack continued. “You can clearly hold that up for a week, or maybe even a month — but the question is, can you hold that up for a year?”

But T officials are hoping that this latest iteration of late-night service remains more popular than the Night Owl program, which provided buses that departed once every 30 minutes and traced the routes of the subway system. The program operated from 2001 to 2005 before being canceled in a round of budget cuts.

During the first year of the Night Owl, ridership averaged about 2,000 people per night, T officials said at the time. By the spring of 2005, average nightly ridership was 655 people.

Scott camped out at Park Street Station on Friday night watching the crowds flowing in and out of the station. The traffic, she said, “had a nice clip to it.”

“It was just like people were going to ride a new line on the train,” Scott said. “They very specifically had come out just because they had been waiting for it to happen.”

The most popular station for late-night service was Park Street, where nearly 2,000 people entered over the two weekend nights. The other most popular stations were Central, Harvard, Haymarket, Kenmore, and Downtown Crossing.

Among the throngs who took advantage of later trains this weekend was Yilu Zhang, 24, of Cambridge. She and a group of friends planned their Friday night with one goal: Venture out to a part of town where they would not normally stay late at night, simply to celebrate their new options.

“I left a friend’s birthday party early just so I could take advantage of the T late at night,” said Zhang, who works as a paralegal. She and her friends took a Red Line train from Central to Park Street, then switched to the Green Line to a bar in Allston. “You don’t really associate 11:30 as a time when you start your journey on the MBTA.”

Zhang acknowledged that Cambridge residents — and other city dwellers — can be insular in their nightlife choices, choosing spots within walking distance or a cheap taxi ride. A late-night trip to Allston, Zhang said, was a rare experience — and something she anticipates doing more often, now that she can take the T.

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